A fuel cell is an electrochemical cell in which the energy of a reaction between a fuel, such as hydrogen, and an oxidant, such as oxygen, is converted directly and continuously into electrical energy. Fuel cells represent an evolving field of technology of pollution-free electricity generation that is expected to compete with traditional methods of creating and distributing electricity. It is also expected to be used in electricity powered cars, trucks and buses.
Proton-exchange fuel cells, also known as PEM (Polymer Electrolyte Membrane) Fuel Cells (“PEMFC”) are low temperature fuel cells which are being developed for transport applications as well as for stationary applications. In PEMFCs, hydrogen is split at the anode (which in practice is a thin layer of catalyst on the polymer membrane's surface) into protons, that travel across the membrane to the cathode (similar or identical to the anode layer) where they combine with oxygen and electrons (which have traveled to the cathode from the anode via an external “load” circuit) to create water, the cell's only product when using pure hydrogen. In order to function, the membrane must conduct hydrogen ions (protons) but not electrons as this would in effect “short circuit” the fuel cell. The amount of water has to be supervised, since excess of water lowers the efficiency of energy production until the point of short circuiting, and shortage of water medium or dehydration of the cell makes the transport of protons from the anode to the cathode difficult.
The most commonly used polymer electrolyte membrane (PEM) in fuel cells is perfluorinated polymer containing sulfonic groups called Nafion. Its special structure provides the polymer with high proton conductivity, chemical stability and mechanical strength. However this membrane suffers from several drawbacks comprising: (a) relative high cost; (b) reduced performance in fuel cells due to the high resistivity of the electrodes/membrane interface; (c) requiring high hydration in order to work effectively; (d) allowing the crossover of methanol from anode to cathode in direct methanol fuel cells (DMFCs) and thus decreasing the performance of these fuel cells; and (e) limiting the work to acidic environment.
DMFCs are fuel cells in which compressed hydrogen is replaced with methanol for outer energy source replenishment, since methanol is easier to handle than hydrogen and produces energy density orders of magnitude greater than even highly compressed hydrogen. Overcoming the drawbacks of hydrogen-based fuel cells, which use water as the conducting medium, and Nafion as the membrane, requires the replacement of hydrogen with methanol and Nafion with another ion-conducting membrane material.
Among the many different proposed membrane alternatives for Nafion is a combination of a polymeric matrix and an ionic liquid. Such composite membrane is described, for example, in WO 2005/045976 in methanol-based fuel cells. This publication, however, fails to demonstrate any contribution of the ionic liquid membrane fuel cells to electrical conduction, especially when compared to cell assemblies where no ionic liquid is used.
It is, therefore, an object of the present invention to provide a PEM (Polymer Electrolyte Membrane) which has high electrolytic conductivity.
It is another object of the present invention to provide a PEM that does not depend on the membrane water content and provides high electrical conductivity even at elevated working temperatures.
It is yet another object of the present invention to provide a PEM that can operate in acidic as well as in basic environments.
It is yet another object of the present invention to provide a PEM without the crossover of methanol through the membrane from the anode to the cathode.
It is yet another object of the present invention to provide a cost-effective low resistivity PEM that can be prepared as a self-standing membrane or as a direct cast on fuel cells electrodes.
It is yet another object of the present invention to provide a method for producing a PEM according to the present invention.
This and other objects of the invention shall become clear as the description proceeds.